Voltage comparator with reduced settling time

ABSTRACT

An improved voltage comparator for use in waveform characterization and like applications is disclosed. The current drawn through two matched transistors connected to a source of current is measured to determine whether a reference signal applied to the base of a first reference transistor is greater or less than the signal to be measured applied to the base of the other transistor. Current is only drawn through the two transistors at times corresponding to desired sampling times, substantially eliminating differential heating effects. The comparator provides substantially reduced settling times, resulting in reduced distortion of the waveform.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved voltage comparator. Moreparticularly, this invention relates to a voltage comparator havingreduced settling time errors and thus reduced distortion.

2. Background of the Invention

There are many occasions throughout electronic engineering, andparticularly in instrument manufacture, where it is necessary to comparea reference voltage to a signal voltage. For example, so-called"sampling comparators", as used in many oscilloscopes, measure thevarying voltage of a signal to provide a suitable display thereof.Sampling voltage tracker circuits using an "equivalent-time" samplingapproach are commonly used in such instruments. See for example U.S.Pat. No. 4,654,484 to Gyles, and "An 8-bit 200 MHz BiCMOS Comparator"Lim et al, IEEE J. of Solid-State Circuits, 25, 1, February 1990.

A block diagram of such a sampling comparator is shown in FIG. 1, andincludes a time delay generator 10, a sampling comparator circuit 12,and a controller 14 with integral waveform display capability. The inputsignal to be displayed f(t) is supplied to the sampling comparatorcircuit 12 and to the time delay generator 10. Controller 14 determinesa point on the input waveform f(t) to be measured. The samplingcomparator circuit 12 generates a reference voltage and compares thereference voltage with the signal f(t) at the same point on eachsuccessive "copy" of the input signal. The comparison is made responsiveto a strobe signal from the time delay generator 10. The samplingcomparator circuit 12 thus provides an output signal to controller 14responsive to the level of the waveform f(t) at the sampling timedefined by the strobe signal. Controller 14 then instructs the timedelay generator to alter the timing of the strobe signal, so that thesampling comparator similarly samples a subsequent point on thewaveform. The process is repeated until the entire waveform has beensampled. At this point the complete series of values for the sampledpoints on the waveform are employed by controller 14 to reconstitute anddisplay the waveform.

More specifically, the reference voltage is compared to the signalwaveform repetitively at the same point in each cycle of the inputwaveform, when strobe pulses are received from a time delay generator10. Over several cycles of operation, the reference voltage will closelyapproximate the input waveform f(t) at that particular point on thewaveform. When successive comparison indicates that the referencevoltage is substantially equal to the signal f(t), or after a fixednumber of cycles of operation, controller 14 stores the referencevoltage, and causes the time delay generator 10 to alter the timing ofthe strobe pulses so that the sampling takes place at a different pointon the waveform. The process is then repeated, measuring a slightlydifferent point on the waveform. When the entire waveform has beensampled at very short intervals, an accurate representation thereof canbe generated and displayed.

FIGS. 2 and 3 illustrate two different embodiments of the samplingcomparison circuit component 12 of the block diagram of FIG. 1. In FIG.2, FIG. 2a shows a block diagram of the circuit, FIG. 2b shows thevoltage of the signal f(t) being measured as a function of time, FIG. 2cshows the sequence of strobe pulses and FIG. 2d shows the referencesignal, which converges to equal the value of the waveform f(t) at thestrobe time. In the circuit of FIG. 2a, the signal to be measured isinput to a comparator 20, together with a reference voltage provided byan integrator 22. The comparison of the signal and reference voltagesoccurs when a strobe signal pulse (FIG. 2c) is received. The successiveoutputs of the comparator 20 are summed in integrator 22, becoming thereference input. As indicated, a digital voltmeter 21 can be used toprovide a digitized output signal, e.g. to controller 14 (FIG. 1).

As shown in FIG. 2b, the strobe pulses of FIG. 2c control the time atwhich the comparison between signal input and the reference inputoccurs. As shown in FIG. 2d the reference signal will typicallyoscillate slightly about the actual value of the signal f(t) at thestrobe time.

FIG. 3 illustrates a further version of a sampling comparator circuit 12which can be used in the instrument of FIG. 1. FIG. 3a is a blockdiagram of the circuit itself. FIG. 3b shows the signal voltage f(t) asa function of time. The signal f(t) is sampled at intervals defined by astrobe signal shown in FIG. 3c. FIG. 3d shows the reference signal. FIG.3e shows the individual bits of a digital word representing the value ofthe reference voltage. One bit of this digital word is output upon eachcomparison.

Referring to FIG. 3a, comparator 20 compares the input signal f(t) to areference signal upon receiving a strobe pulse (FIG. 3c) and outputseither a "one" or a "zero" bit responsive to the comparison. Asuccessive approximation register (SAR) 23 receives the output bits, andprovides a digital word as input to a digital-to-analog converter (DAC)24. DAC 24 increments or decrements the reference voltage applied to thereference input of the comparator 20 responsive to the word provided bySAR 23 after the previous comparison. As shown in FIG. 3d, the change inthe reference voltage responsive to each bit output by the successiveapproximation register has half the value of the change responsive tothe preceding bit. Accordingly, the most significant bit of the word isoutput first, the second most significant bit is output next, and so onas indicated in FIG. 3e. Thus, at the conclusion of n samplingintervals, the digitized output provided by the successive approximationregister 23 is an n-bit digital word directly representative of thevalue of the input signal at the strobe time.

Essentially the same comparator 20 is used in the circuits of both FIGS.2 and 3. In the prior art, such comparators include two identicaltransistors, typically formed on the same substrate for uniformity, andconnected differentially to a source of current. The reference voltagesignal is applied to the base of a first reference transistor and theinput signal to be measured is applied to the base of a second signaltransistor. Accordingly, the amount of current conducted through therespective transistors can be measured and compared to determine whetherthe reference signal is greater than the sample signal or vice versa.Typically, the currents conducted through the two transistors arelatched by a second pair of transistors responsive to the strobe pulse.See the Lim et al paper referred to above.

As discussed above, in the sampling circuits of FIGS. 2 and 3, thecomparison of the reference voltage to the signal voltage takes place ata single instant during each period of the signal. Conventionally,however, the signal voltage and the reference voltage are provided tothe bases of the signal and reference transistors of the comparatorthroughout the waveform, except at the strobe time, when the power isapplied to the latching transistors. See Lim et al. Accordingly, thesignal and reference transistors conduct varying amounts of currentcorresponding to the different levels of the input signal f(t) and thereference voltage. Therefore, differing quantities of heat aredissipated by the two transistors of the comparator. Accordingly, eventhough as noted both transistors are commonly formed on a singlesubstrate, as they are heated differently their base-to-emitter voltagecharacteristics vary somewhat inconsistently,

Such differential heating effects are known to the art to cause a slightinaccuracy in the comparison and to distort the signal as finallyrepresented by a series of words determined as above. For example, the"corners" of a square wave input signal f(t) tend to be rounded due tosuch differential heating effects. This distortion is referred to as a"thermal tail". U.S. Pat. No. 4,807,147 to Halbert et al, e.g., atColumn 2, line 12, refers to such thermal tails. Halbert however doesnot provide any solution to thermal tails caused by the phenomenon justdiscussed. This distortion is particularly evident at input signalfrequencies corresponding to the thermal time constants exhibited by thetransistors of the comparator circuit.

The art suggests that such thermal tails may be compensated for insoftware; for example, controller 14 may be provided with software tocorrect the distortion induced by the thermal tails. However, suchsoftware compensation is complex and is only useful in a narrow range offrequencies.

The prior art also teaches preamplifying the input waveform f(t) and thereference signal using a second differential pair of transistors.Thermal tail phenomena are also understood to originate in suchpreamplifying transistors, again due to differential heating effects.

OBJECTS AND SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a voltagecomparator for comparing a reference signal to an input signal at aparticular instant of time, such that thermal tails distorting thereconstructed waveform are avoided, and accordingly to provide a signalsampling circuit having reduced distortion as compared to the prior art.

This object of the invention and others which will appear as thediscussion below proceeds are satisfied by the present invention whereina comparator comprising two transistors, the base of one transistorbeing connected to the input signal and the base of the other transistorbeing connected to the reference signal, is only connected to a sourceof current at specific times closely corresponding to the strobe timesat which the reference signal is actually compared to the input signal.In this way, differential heating of the two transistors issubstantially eliminated. Where the comparator circuit incorporatespreamplification, the preamplifying transistors can similarly bedisconnected from the power source except at the time the measurement isto be made.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood if reference is made to theaccompanying drawings, in which:

FIG. 1 as discussed above is a block diagram of a waveformreconstruction and display instrument incorporating a samplingcomparator circuit according to the prior art;

FIG. 2, comprising FIGS. 2a-2d, illustrates a sampling voltage trackerelement as used in the prior art, wherein FIG. 2a is a schematic diagramof the sampling voltage tracker and FIGS. 2c -2d show related waveformsas functions of time;

FIG. 3, comprising FIGS. 3a-3e, similarly illustrates a secondembodiment of a sampling voltage tracker element used in the prior art,with related waveforms;

FIG. 4 is a functional block diagram of the improved comparatoraccording to the invention;

FIG. 5 is a more detailed, partly schematic diagram of the comparatorcircuit according to the invention;

FIG. 6 is a graph of a reconstructed square-wave input signalillustrating distortion induced by thermal tail phenomena inherent inthe comparator circuits according to the prior art; and

FIG. 7 is a graph comparable to FIG. 6 illustrating the substantiallyimproved waveform reconstruction provided by use of the circuit of theinvention; and

FIG. 8 is a timing diagram showing the relationship of enable and strobesignals encountered in operation of the comparator of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As indicated above, FIG. 1 illustrates instruments in the prior art forproviding accurate reconstruction and display of a cyclical input signalf(t), using one of the sampling voltage comparator circuits of FIGS. 2or 3 to measure the waveform at successive points thereon. The presentinvention includes an improved comparator which may be substituted forthe comparator 20 in either of the circuits of FIGS. 2a and 3a. FIG. 4is a block diagram of a comparator according to the invention, and FIG.5 is a more detailed, partially schematic block diagram thereof.

In the preferred embodiment, the reference signal voltage V_(ref) andthe signal f(t) to be measured, V_(sig), are supplied to a preamplifier25 made up of a matched pair of transistors 26 and 27, and then to adifferential amplifier 32 made up of a second matched pair oftransistors 34 and 36. More specifically, after pre-amplificationV_(sig) is supplied to the base of a first signal transistor 34, and thereference voltage V_(ref) is supplied to the base of the secondreference transistor 36. Transistors 34 and 36 are differentiallyconnected through resistances 38 to a source of potential V+ indicatedat 40. When the circuit through a current source 42 is completed,currents proportional to the voltages present at the bases of thetransistors 34 and 36 flow therethrough. In a generalized embodimentshown in FIG. 4, current flows only when a current switch 46 is closedresponsive to an enable input 60. In the preferred embodiment, shown inFIG. 5, the function of current switch 46 is performed by switchingvoltage controlled current source 42 to turn on responsive to enableinput 60. Enable input 60 is provided just prior to strobe input 62;strobe input 62 is provided to a second current switch 44, whichcontrols the time at which the input signal is sampled.

FIG. 8 shows the relationship of the enable input 60 and the strobeinput 62; as indicated by FIG. 8, the enable input precedes the strobeinput by approximately 10nanoseconds, defining a sampling period. Thesampling period is thus kept short to prevent differential heating oftransistors 34 and 36, and reducing the settling time of the comparatoraccording to the invention.

More specifically, when the strobe input 62 is received, theinstantaneous potential across the collectors of transistors 34 and 36is stored in a latching circuit 50 made up of another pair oftransistors 52 and 54. The comparison signal provided thereby isprovided to emitter coupled logic (ECL) interface 56 to provide suitablelogic level output signals. Thus ECL interface 56 may be connected tointegrator 22 in the circuit of FIG. 2 or successive approximationregister 23 in the circuit of FIG. 3.

As indicated in FIG. 5, the current switch 44 is switched between a"track" position, wherein the comparison is made, and a "latch" positionwhere the result of the comparison is latched. See Lim et al, supra.According to the invention, just prior to the time it is desired tosample the input signal, e.g. 10 ns earlier, an enable signal 60 isprovided to the current source 42.

See FIG. 8, as discussed above, illustrating the relationship of theenable input and the strobe input together defining the sampling period.In this way, according to the invention, current is only drawn throughthe signal and reference transistors 34 and 36 respectively when it isactually desired to sample the waveform. The overall heating experiencedby the transistors 34 and 36 is therefore very greatly reduced.

As noted above, it is commonly desired to provide pre-amplification ofthe input signal f(t) and the reference signal in preamplifier stage 25,including transistors 26 and 27. Transistors 26 and 27 likewiseexperience differential heating effects and contribute to the thermaltail phenomenon. Therefore, according to a further aspect of theinvention, a further voltage controlled current source 28 supplyingtransistors 26 and 27 is controlled by a further enable signal 29 tosimilarly turn on only when it is desired to compare the input andreference signals, e.g. just prior to the strobe pulse 62. In most casesenable input 29 and enable input 60 may be identical.

In a further embodiment of the invention, the current source 28 and 42may instead be deactivated by disconnecting the negative supply voltageV- from the entire circuit, as indicated by optional switch 66. In thisembodiment, voltage controlled current sources 28 and 42 would bereplaced by current sources operating when in circuit with anappropriate source of potential and load. However, in mostimplementations this would likewise disconnect V- from the remainder ofthe associated circuitry (e.g. the ECL interface 56), which might leadto further complications.

FIGS. 6 and 7 show dramatically the improvement provided according tothe present invention. FIG. 6 is a waveform depicting voltage as afunction of time illustrating the response of the prior art comparatorto an input square wave, that is, wherein the reference voltage issupplied to the reference transistor and the signal voltage is suppliedto the signal transistor continually. It will be observed that the priorart circuit requires some 150 ns to settle to its final value, and thatthe square-wave input waveform is distorted to assume a rounded shape.FIG. 7 depicts the response of the same circuit incorporating theimprovement according to the invention. Thus, in FIG. 7, current onlyflows through the transistors at approximately the strobe sampling time,and the output settles to its final value in less than 25 ns. Thereforeit is apparent from comparison of FIGS. 6 and 7 that substantiallyimproved results are provided according to the invention.

The improvement of the invention may be employed in a wide variety ofcircuits requiring accurate voltage measurements, such as digitaloscilloscopes and signal waveform recorders, and may also be useful incharacterization of waveform generators and like instruments. Theimproved response of the comparator according to the invention isparticularly useful in accurately characterizing step-like orsquare-wave signals requiring very fast, undistorted response. Thecircuit of the invention also more accurately characterizes sine wavesignals, particularly as to rms measurements.

Implementation of the invention is within the skill of the art. Theresults shown in FIG. 7 were obtained upon test of a prototypicalversion of an application-specific integrated circuit (ASIC) fabricatedto the inventors' design. The ASIC was disposed in a signal probe andwas connected as closely as possible to the input signal lead. Theenable input signal 60 was obtained by a simple modification of the timedelay generator 10 (FIG. 1). Useful results were also demonstrated bydisconnecting V- from the current sources, as indicated by optionalswitch 66 (FIG. 5).

As indicated in FIGS. 4 and 5, the switch 44 responsive to the strobeinput 62 that controls whether current flows through the differentialamplifier 38 or the latch 50 is preferably configured as a Schmitttrigger, to provide the fastest possible switching speed. Switch 46,connecting the current source 42 to the differential amplifier 32responsive to the enable input 60, could also be a Schmitt trigger, butneed not be, as the performance requirements are relatively low. In onesuccessfully tested implementation of the invention, current source 42was provided by a transistor connected between a circuit reference pointV- and the Schmitt trigger current switch 44, and having enable inputsignal 60 applied to its base. As noted, V- could equivalently bedisconnected from the circuit except during the desired measurementinterval.

It will be appreciated that differential heating of the reference andsignal transistors could also be eliminated by disconnecting the V_(ref)and V_(sig) input signals from the bases of the transistors.Disconnecting the bases of the corresponding transistors for differentpurposes is discussed by Lim et al, supra, at p. 196. As acknowledged byLim et al, this would involve substantial potential for erroneousmeasurements due to transient noise, impedance effects and the like.Accordingly, solution of the thermal tail problem according to theinvention as described above is preferred.

More particularly, the enabling technique of the invention not onlyreduces thermal errors but can reduce any "low frequency" aberration(thermal or electrical) in the transfer function of the comparator. Thepresence of undesirable electrical time constants (a linear systemsphenomenon) within prior art comparators also can add tails to thesettling performance. In either case, the problem is that thecomparator's response time is too long. The enabling procedure of theinvention for eliminating "thermal tails" also applies to these impulseresponse tails. In effect, the time difference, t_(d), between theactivation of the current source and the leading edge of the strobepulse defines the effective response time of the comparator. Any tailsin the impulse response of the comparator that extend beyond thisduration will effectively be truncated. In the frequency domain, thefrequency response of the comparator will be the Fourier transform ofthe truncated impulse response. The truncation will be manifested as atendency to flatten the frequency response or transfer function forfrequencies less than 1/t_(d).

Inasmuch as the present invention is subject to many variations,modifications and changes in detail, it is intended that all subjectmatter discussed above or shown in the accompanying drawings beinterpreted as illustrative only and not be taken in a limiting sense.

What is claimed is:
 1. In a voltage comparator circuit comprising a pairof transistors, each having an emitter, a base and a collector, each ofsaid transistors being connected to conduct current along anemitter-collector current path from a first current source to areference point, the base of a first reference transistor of said pairbeing connected to means for supplying a reference voltage level, andthe base of a second signal transistor of said pair being connected tomeans for supplying a signal the voltage level of which is to bemeasured, and means for comparing the currents conducted through thefirst and second transistors to compare the reference voltage level tothe signal voltage level, the improvement comprising:switching means inthe emitter-collector current path of each of said pair of transistors,for selectively controlling the flow of current from said source ofcurrent through said first and second transistors; and timing means forcontrolling said switching means in order that current flows throughsaid first and second transistors only at particular times when it isdesired to compare the reference voltage level to the signal voltagelevel.
 2. The improvement of claim 1, wherein said voltage comparatorcircuit is part of an instrument for measuring the voltage level of arepetitive signal at regular intervals of time, wherein said referencevoltage level is compared to said signal voltage level at sampling timesspaced by said intervals of time, and said timing means controls saidswitching means such that current flows through said transistors only attimes corresponding to said sampling times.
 3. The improvement of claim1, wherein said reference signal level is supplied from an integratorconnected to receive and sum voltages responsive to the output of saidmeans for comparing.
 4. The improvement of claim 1, wherein saidreference signal is supplied from a digital-to-analog converterreceiving a digital word input representative of said sample voltagelevel from a successive approximation register.
 5. The improvement ofclaim 4, wherein the value of each bit of said digital word is setresponsive to prior comparison of said reference signal voltage level tosaid sampled signal voltage level.
 6. The improvement of claim 1,wherein said switching means controls activation of said first source ofcurrent.
 7. The improvement of claim 1, wherein said voltage comparatorcircuit further comprises a preamplifier for preamplifying said inputsignal and said reference voltage level prior to supply to said signaland reference transistors and a second current source for powering saidpreamplifier, and said improvement further comprises switching meanssuch that current flows from said second current source through saidpreamplifier only at said particular times.
 8. A voltage comparatorcircuit comprising:a first source of current; a pair of transistors,each comprising an emitter, a base, and a collector, said transistorseach being connected to conduct current along an emitter-collectorcurrent path from said first source to a reference point, the amount ofcurrent conducted by each of said transistors varying with voltageapplied to the respective bases thereof, the base of a first referencetransistor being connected to a reference voltage level and the base ofa second signal transistor being connected to a signal the voltage levelof which is to be measured; means for comparing the currents conductedthrough the first and second transistors to compare the referencevoltage level to the signal voltage level, and for providing an outputsignal responsive to said comparison; switching means in theemitter-collector current path of each of said first and secondtransistors for controllably connecting said source of current to saidfirst and second transistors; and timing means for controlling saidswitching means to effectively connect said first source of current tosaid first and second transistors only at particular times when it isdesired to compare the reference voltage level to the signal voltagelevel.
 9. The voltage comparator circuit of claim 8, in combination withmeans for recording values of the voltage of a repetitive signalmeasured at regular intervals of time, said measured values beingresponsive to said output signal, wherein said reference voltage levelis compared to said signal voltage level at sampling times spaced bysaid intervals of time, and said timing means controls said switchingmeans to connect said source of current to said transistors at timescorresponding to said sampling times.
 10. The voltage comparator circuitof claim 8, further comprising an integrator for supplying saidreference signal, said integrator being connected to receive and sumvoltages responsive to the output of said means for comparing.
 11. Thevoltage comparator circuit of claim 8, further comprising a successiveapproximation register and a digital-to-analog converter, saiddigital-to-analog converter receiving a digital word inputrepresentative of said sample voltage level from said successiveapproximation register, and supplying said reference signal to saidfirst reference transistor.
 12. The voltage comparator circuit of claim11, wherein the value of each bit of said digital word is set responsiveto the comparison of said reference signal voltage level to said sampledsignal voltage level.
 13. The voltage comparator circuit of claim 8,wherein said switching means controls activation of said source ofcurrent.
 14. The voltage comparator circuit of claim 8, furthercomprising a preamplifier for preamplifying said input signal and saidreference voltage level prior to supply to said signal and referencetransistors, a second current source for powering said preamplifier, andswitching means for controlling flow of current from said second currentsource through said preamplifier only at said particular times.